1. Field of the Invention
This invention relates to a method of vacuum processing of materials to reduce their optical reflection, and in the case of transparent substrates of increasing their optical transmission. It uses a random but controlled etch mask of globular material to form vacuum-etched microstructures in surfaces.
By suitably recoating the globular layer and substrate at an intermediate stage of the etching process, it is possible to fabricate surfaces with anisotropy which are useful in the crystallisation of thin amorphous films.
2. Description of the Prior Art
The optical properties of graded-index layers have been investigated, both experimentally and theoretically, for some time. By graded-index is meant that the optical refractive index varies in a monotonic fashion between two optical materials, such as between silicon and air. The index may vary stepwise, as would be the normal case with evaporated thin films, or continuously over some values of refractive index; however in general it has been found that graded films can have desirable performance over a wide range of incident wavelengths and angles. Theoretical work on graded layers has been done by R. Jacobsson, in Progress in Optics, edited by E. Wolf (North-Holland, Amsterdam, 1966), Chapter V, pages 247 to 286, and the application of such graded layers to high-index substrates such as germanium is described by J. A. Dobrowolski, in Handbook of Optics, edited by W. C. Driscoll and W. Vaughan (McGraw-Hill, New York, 1978), Chapter 8, pages 8-54 to 8-56. Unfortunately thin-film coating materials suitable for grading glass or quartz, which have a relatively low refractive index, to air, are not available since materials of extremely low refractive index would necessarily be tenuous and fragile. Materials of such low index have been graded by several methods in the past, all of them involving the use of wet (in liquid or fume form) chemical reagents which attack some constituents of the surface of the material. Depending on the heat treatment of the material and on the etching bath, various degrees of grading have been achieved. Examples of glass etching methods can be found in F. H. Nicol, RCA Review, Vol. 10, no. 3,
September 1949, pages 440 to 447, and M. J. Minot, Journal of the Optical Society of America, Vol. 66, no. 6, June 1976, pages 515 to 519, and Vol. 67, no. 8, August 1977, pages 1046 to 1050.
Another case in which grading is desirable but difficult is that of metals. It is often desirable to make metallic surfaces highly absorbing over fairly broad, but specific optical wavelength ranges, for instance for solar-selective surface fabrication (for a general description of some solar-selective surfaces, see A. B. Meinel and M. P. Meinel, Physics Today, Vol. 25, no. 2, February 1972, pages 44 to 50). Suitable grading methods up to now have involved the deposition of graded composite (metal-ceramic mixture) materials or the formation of microstructures on the metal surface, sometimes by vacuum sputtering in a contaminated environment. Examples of such sputtered microstructures can be found in J. L. Vossen, Journal of Vacuum Science and Technology, Vol. 8, no. 5, 1971, pages S12 to S30, on page S22, where backing-plate contamination caused varying microstructures to be formed on a silicon surface. In addition, W. R. Hudson, Journal of Vacuum Science and Technology, Vol. 14, no. 1, Jan/Feb 1977, pages 286 to 289 shows how contamination ("seed") material was sputtered onto a surface while the surface was being sputter-etched, giving structures which depended greatly on the surface temperature, surface material, "seed" material, and position of the "seed" source. Another similar reference is R. S. Berg and C. J. Kominiak, Journal of Vacuum Science and Technology, Vol. 13, no. 1, 1976, pages 403 to 405, where microstructural layers on surfaces were formed by sputter-etching through contamination layers already present on the surfaces, and through layers, formed on the surfaces while sputter-etching, which probably originated from the surface-support table. All of these methods resulted in surfaces of widely variable, highly process-dependent structures, and none of these methods is suitable for the direct formation of anti-reflecting, highly transmitting graded layers.
In the present context, sputter-etching is taken to mean the removal of material from a surface which is bombarded by energetic species (atoms, molecules, and their ionized forms) in a vacuum system to which is introduced a gas or mixture of gases. These energetic species result from an electric discharge inside the vacuum system, and may be formed directly above the surface, or in an "ion gun" at some distance from the surface. The term "reactive sputter-etching" refers to sputter-etching where the bombarding species is chemically reactive with some of the surface materials in such a fashion that the surface material is largely prevented from redepositing on the surface, and is instead carried away from the sputtering region by the pumped flow of the gases employed. Examples of reactive sputter-etching gases are: for silicon and silicon compounds; hydrogen, carbon tetrafluoride and other chloro-fluorocarbons, and their mixtures with oxygen: for tin and tin compounds: hydrogen and mixtures of hydrogen and oxygen. The effects of reactive sputter-etching, as opposed to those of standard sputter-etching, are well described in several papers; for instance H. W. Lehmann and R. Widmer, Journal of Vacuum Science and Technology, Vol. 15, no. 2, March/April 1978, pages 319 to 326, and N. Hosokawa, R. Matsuzaki, and T. Asamaki, Proc. 6th International Vacuum Congress, 1974; Japanese Journal of Applied Physics Supplement 2, part 1, 1974, pages 435 to 438. These papers describe a form of reactive sputter-etching which is the preferred embodiment for the present invention, but which is not the only possible method. This form has the incident energetic species approaching approximately normally incident to the surface to be etched, however here we will also allow isotropic incidence, a case which is often referred to in the literature as "plasma etching".
Here we also define a globular layer as a layer which is formed by the deposition of a material onto a surface such that the material self-agglomerates into a layer of non-uniform thickness. That is, the internal forces of the globular layer material combined with the structure and interaction forces of the surface, act to cause the layer material to form in a non-uniform fashion on the surface. The thickness varies on a microscopic scale so that the surface appears to be rough under microscopic examination. For instance, the spatial period of thickness variations, though inherently random, may be of the order of 100 micrometers to one nanometer. Globular layers may be formed as separated islands of material or as closely contiguous protruberances. One possible method of forming globular surfaces is the in-situ polymerisation of a monomer to form closely spaced spherical particles on the surface: this method is actually well known to produce particles of exceptional regularity. globular layers can also be formed from metals of relatively low melting point by evaporation of a thin metal layer onto a heated surface. Examples of such globular metal layers are in C. M. Horwitz, R. C. McPhedran, and J. A. Beunen, Journal of the Optical Society of America, Vol. 68, 1978, pages 1023 to 1031, and C. M. Horwitz, Applied Physics Letters, Vol. 36, no. 9, May 1, 1980, pages 727 to 730. It is often necessary to deposit an intermediate layer between the surface and the globular layer in order to assure proper globular layer formation, but this in no way affects the basis of the following invention.
Anisotropic surface structures, that is structures which have orientation directions in the surface plane, have been shown to be of value for the crystallisation of semiconductors see for instance M. W. Geiss, D. C. Flanders, H. I. Smith, Applied Physics Letters Vol. 35, July 1, 1979, pages 71 to 74, in which a regular anisotropic surface was used in the crystallisation of a silicon surface. Up to the present, such anisotropic surfaces have required complex and expensive processing to attain the fine structures required.